Objective-C

Objective-C is the primary language used to write Mac software. If you're comfortable with basic object-oriented concepts and the C language, Objective-C
will make a lot of sense. If you don't know C, you should read the
C Tutorial first.

You can call methods on classes too, which is how you create objects. In the
example below, we call the string method on the NSString class, which
returns a new NSString object:

id myObject = [NSString string];

The id type means that the myObject variable can refer to any kind of object,
so the actual class and the methods it implements aren't known when you compile the app.

In this example, it's obvious the object type will be an NSString, so we can change the type:

NSString* myString = [NSString string];

This is now an NSString variable, so the compiler will warn us if we try to use
a method on this object which NSString doesn't support.

Notice that there's a asterisk to the right of the object type. All Objective-C object variables are
pointers types. The id type is predefined as a pointer type, so there's no need
to add the asterisk.

Nested Messages

In many languages, nested method or function calls look like this:

function1 ( function2() );

The result of function2 is passed as input to function1. In Objective-C, nested messages
look like this:

[NSString stringWithFormat:[prefs format]];

Avoid nested nesting more than two message calls on a single line, as it easily gets unreadable.

Multi-Input Methods

Some methods take multiple input values. In Objective-C, a method name can be split
up into several segments. In the header, a multi-input method looks like this:

These are not just named arguments. The method name is actually writeToFile:atomically: in the runtime system.

2

Accessors

All instance variables are private in Objective-C by default, so you should use
accessors to get and set values in most cases. There are two syntaxes. This is the traditional 1.x syntax:

[photo setCaption:@"Day at the Beach"];
output = [photo caption];

The code on the second line is not reading the instance variable directly.
It's actually calling a method named caption. In most cases, you don't
add the "get" prefix to getters in Objective-C.

Whenever you see code inside square brackets, you are sending
a message to an object or a class.

Dot Syntax

The dot syntax for getters and setters is new in Objective-C 2.0, which is part of Mac OS X 10.5:

photo.caption = @"Day at the Beach";
output = photo.caption;

You can use either style, but choose only one for each project.
The dot syntax should only be used setters
and getters, not for general purpose methods.

3

Creating Objects

There are two main ways to create an object. The first is the one you saw before:

NSString* myString = [NSString string];

This is the more convenient automatic style. In this case, you are creating an
autoreleased object, which we'll look at in more detail later. In many cases,
though, you need to create an object using the manual style:

NSString* myString = [[NSString alloc] init];

This is a nested method call. The first is the alloc method called on NSString itself. This
is a relatively low-level call which reserves memory and instantiates an object.

The second piece is a
call to init on the new object. The init implementation usually does basic setup, such as
creating instance variables. The details of that are unknown to you as a client of the class.

In some cases, you may use a different version of init which takes input:

NSNumber* value = [[NSNumber alloc] initWithFloat:1.0];

4

Basic Memory Management

If you're writing an application for Mac OS X, you have the option to enable garbage collection.
In general, this means that you don't have to think about memory management until you get to
more complex cases.

However, you may not always be working with an environment that supports garbage collection. In that
case, you need to know a few basic concepts.

If you create an object using the manual alloc style, you need to release the
object later. You should not manually release an autoreleased object because your
application will crash if you do.

Add Methods

Remember, Objective-C methods typically leave out the "get" prefix.
A single dash before a method name means it's a instance method. A plus
before a method name means it's a class method.

By default, the compiler assumes a method returns an id object, and that
all input values are id. The above code is technically correct, but it's unusual.
Let's add specific types for the return values:

Each setter deals with two variables. The first is a reference to the
existing object, and the second is the new input object. In a garbage
collected environment, we could just set the new value
directly:

- (void) setCaption: (NSString*)input {
caption = input;
}

But if you can't use garbage collection, you need to release the old
object, and retain the new one.

There are actually two ways to free a reference to an object: release and
autorelease. The standard release will remove the reference immediately.
The autorelease method will release it sometime soon, but it will definitely stay
around until the end of the current function (unless you add custom code to
specifically change this).

The autorelease method is safer inside a setter because the variables for the new
and old values
could point to the same object. You wouldn't want to immediately release an object which
you're about to retain.

This may seem confusing right now, but it will make more
sense as you progress. You don't need to understand it all yet.

Init

We can create an init method to set inital values for our instance variables:

On the first two lines, we just send release to each of the instance variables.
We don't need to use autorelease here, and the standard release is a bit faster.

The last line is very important. We have to send the message[super dealloc] to
ask the superclass to do its cleanup. If we don't do this, the object will not
be removed, which is a memory leak.

The dealloc method is not called on objects if garbage collection is enabled.
Instead, you implement the finalize method.

7

More on Memory Management

Objective-C's memory management system is called reference counting. All you have
to do is keep track of your references, and the runtime does the actual freeing
of memory.

In simplest terms, you alloc an object, maybe retain
it at some point, then send one release for each alloc/retain you sent.
So if you used alloc once and then retain once, you need to release twice.

That's the theory of reference counting. But in practice, there are usually only two reasons to
create an object:

1. To keep it as an instance variable
2. To use temporarily for single use inside a function

In most cases, the setter for an instance
variable should just autorelease the old object, and retain the new one. You then
just make sure to release it in dealloc as well.

So the only real work is managing local references inside a function. And there's
only one rule: if you create an object with alloc or copy,
send it a release or autorelease message at the
end of the function. If you create an object any other way, do nothing.

Notice how the rules for managing local references are exactly the same,
regardless of whether you're setting them as instance variables or not.
You don't need to think about how the setters are implemented.

If you understand this, you understand 90% of what you will ever need to
know about Objective-C memory management.

8

Logging

Logging messages to the console in Objective-C is very simple. In fact,
the NSLog() function is nearly identical to the C printf() function, except
there's an additional %@ token for objects.

NSLog ( @"The current date and time is: %@", [NSDate date] );

You can log an object to the console. The NSLog function calls the description method
on the object, and prints the NSString which is returned. You can override the description
method in your class to return a custom string.

9

Properties

When we wrote the accessor methods for caption and author earlier,
you might have noticed that the code is straightforward, and could probably be generalized.

Properties are a feature in Objective-C that allow us to automatically generate
accessors, and also have some other side benefits. Let's convert the Photo class to
use properties.

The @property is an Objective-C directive which declares the property. The
"retain" in the parenthesis specifies that the setter should retain the input
value, and the rest of the line simply specifies the type and the name of the
property.

The @synthesize directive automatically generates the setters and getters for
us, so all we have to implement for this class is the dealloc method.

Accessors will only be generated if they don't already exist, so feel free to specify @synthesize
for a property, then implement your custom getter or setter if you want. The compiler will
fill in whichever method is missing.

There are many other options for the property declarations, but those are
outside of the scope of this tutorial.

10

Calling Methods on Nil

In Objective-C, the nil object is the functional equivalent to the NULL
pointer in many other languages. The difference is that you can call methods
on nil without crashing or throwing an exception.

This technique used by the frameworks in a number of different ways, but the
main thing it means to you right now that you usually don't need to check
for nil before calling a method on an object. If you call a method on nil that
returns an object, you will get nil as a return value.

This works because when we set nil as an instance variable, the setter just retains
nil (which does nothing) and releases the old value. This approach is often better
for dealloc because there's no chance of the variable pointing at random data where
an object used to be.

Note that we're using the self.<var> syntax here, which means
we're using the setter and picking up the memory management for free. If
we just directly set the value like this, there would be a memory leak:

Categories

Categories are one of the most useful features of Objective-C. Essentially,
a category allows you to add methods to an existing class without subclassing
it or needing to know any of the details of how it's implemented.

This is particularly useful because you can add methods to built-in objects. If
you want to add a method to all instances of NSString in your application, you
just add a category. There's no need to get everything to use a custom subclass.

For example, if I wanted to add a method to NSString to determine if the contents
is a URL, it would look like this:

This is very similar to a class declaration. The differences are that
there is no super class listed, and there's a name for the category in
parenthesis. The name can be whatever you want, though it should communicate
what the methods inside do.

Here's the implementation. Keep in mind this is not a good implementation
of URL detection. We're just trying to get the concept of categories across:

Unlike subclasses, categories can't add instance variables. You can, however, use
categories to override existing methods in classes, but you should do so
very carefully.

Remember, when you make changes to a class using a category, it affects all instances
of that class throughout the application.

Wrap Up

This is a basic overview of Objective-C. As you've seen, the language is
pretty easy to pick up. There's not much special syntax to learn, and the same conventions are
used over and over again throughout Cocoa.

If you'd like these examples in action,
download the project below and look through the source code: